1. Technical Field
The present invention relates to semi-insulating gallium nitride baseplates and epitaxial substrates in which such gallium nitride baseplates are utilized, and to methods of forming gallium nitride into which iron has been introduced.
2. Description of the Related Art
In Physica Status Solidi (a), Volume 200, Issue 1, 2003, pp. 18-21, an iron-doped GaN material is grown on a sapphire substrate using a hydride vapor phase epitaxy (HVPE) tool. An iron dopant is supplied in the form of iron chloride, which is formed by flowing HCl on metallic iron. The GaN sample having the highest resistance has an iron concentration of 4×1016 cm−3.
In Applied Physics Letters, Vol. 81, No. 3, 15 Jul., 2002, pp. 439-441, an iron-doped GaN layer is grown by metalorganic vapor phase deposition (MOCVD). An iron dopant is supplied in the form of ferrocene (Cp2Fe: bis(cyclopentadienyl)iron) using a carrier gas (hydrogen). A GaN layer having a resistivity of 7×109 Ω/sq has a thickness of 2.6 micrometers.
Japanese Unexamined Pat. App. Pub. No. H11-40897 describes an InP semiconductor laser device. The semiconductor laser device has an iron-doped high-resistance InP semiconductor layer.
In recent years, attention has been given to GaN electronic devices as materials for achieving high-speed and high-breakdown-voltage devices. In general, a sapphire substrate and a silicon carbide (SiC) substrate are used as substrates for fabrication of the GaN electric device. It is essential to employ a semi-insulating substrate to reduce stray capacitance associated with the substrate, in order to achieve high-speed operation of the GaN electric device.
The above-described substrates have lattice constants which are significantly different from the lattice constants of an AlGaN layer and a GaN layer. For this reason, a number of dislocations occur in these semiconductor layers. Therefore, in order to fabricate a higher-breakdown-voltage device, it is preferable that a substrate (e.g., a GaN substrate or an AIN substrate) having a lattice constant close or equal to that of the above-described semiconductor layer be used. Concerning the GaN substrate, a conductive substrate has at last been made commercially available, but a semi-insulating GaN substrate is not yet available. Both the semi-insulating GaN and AIN substrates are under development.
In the above-cited Physica Status Solidi (a) article, an iron dopant is supplied in the form of iron chloride produced by flowing HCl on metallic iron. In this technique, iron oxide is present on the surface of the metallic iron, so that the reaction of hydrochloric acid and iron is unstable. This method should be modified in order to achieve stable fabrication. If H2 is present in the carrier gas, the equilibrium of the following reaction is shifted to the left side, interfering with the formation of FeCl2.Fe(s)+2HCl(g)=FeCl2(g)+H2(g)
Therefore, as the carrier gas, an inert gas, such as N2 or the like, should be used. On the other hand, metallic iron has a purity of about 5 N. When the metallic iron is used as an iron source, other impurities are doped along with iron.
In Pat. App. Pub. No. H11-40897, a high-resistance InP semiconductor film into which iron has been doped is grown by metalorganic vapor phase deposition using ferrocene. Also, in the Applied Physics Letters article, MOCVD is used to supply ferrocene into a reaction furnace using hydrogen carrier gas. In MOCVD, heating is performed only in the vicinity of a substrate.
However, when a thick GaN film is deposited on a substrate by VPE in which heating is performed using an external heater, ferrocene reacts with hydrogen in accordance with the following reaction.Cp2Fe(g)+H2(g)→2C5H6(g)+Fe(g)
Iron generated by this decomposition forms droplets, so that the iron dopant does not appropriately reach the substrate. For this reason, it is not possible to fabricate a thick gallium nitride film for a semi-insulating GaN substrate having a high resistivity by iron doping.